In recent years, with the development of image sensors having small size and low power consumption, portable devices with a built-in camera, such as PDAs (Personal Digital Assistants), portable telephones, and portable PCs (personal computers) have been in widespread use.
These portable devices are capable of photographing images and printed materials instead of taking notes, for example, using a camera such as a built-in CCD camera. Also, they are capable of storing such photographed images in the portable devices and transmitting the photographed images to other portable devices and PCs through the Internet via an electronic mail function of the portable devices.
Cameras built in such portable devices are manufactured such that they are as small and light in weight as possible. In general, cameras having resolution lower than that of normal digital cameras, namely, about 100 thousand to 300 thousand effective pixels are used.
Also, cameras built in portable terminal devices involve severe limitation in terms of a case size and costs, so that, in general, an automatic focus device is not built in. When a user photographs an image using such a built-in camera, an image in the photographing range of the built-in camera is displayed on a display screen, such as an LCD (liquid crystal display) disposed in a portable terminal device. Thus, it is possible to photograph adjusting the focus while confirming the display screen.
In order to adjust the focus upon photographing, although the focusing may be performed while the user confirms the display screen as mentioned above, the focusing may also be performed such that a focus state is displayed by the portable terminal device, while the user confirms the display of the focus state as well as the display screen. The portable terminal device can comprise a function for displaying the focus state on the basis of the signal components of an image received using the built-in camera.
An imaging apparatus described in Patent Document 1 (JP Patent Publication (Kokai) No. 3-11884 A (1991)) relates to an imaging apparatus such as a video camera having a viewfinder and focus control means. The apparatus comprises a focus state display apparatus for displaying a focus state upon manual focusing based on the ratio of high frequency components to low frequency components of photographed image signals.
A focus state display apparatus and the microscope apparatus described in Patent Document 2 (JP Patent Publication (Kokai) No. 2002-196225 A) comprises a focus state display apparatus by which the high-frequency components of spatial frequencies of image data are extracted to generate differential image data, coloration is changed in accordance with pixel values indicating the high-frequency components of the data, and a focus state is notified using the difference of color.